This application claims the priorities of Japanese Patent Applications No. 7-323975 and No. 7-323976 both filed on Nov. 17, 1995.
In recent years alkaline storage batteries assembled by using a metal compound, such as nickel hydroxide, as the positive electrode material and the new material hydrogen absorbing alloy as the negative electrode material attracted attention as the next-generation alkaline storage batteries replacing nickel-cadmium storage batteries because of their high energy densities per unit weight and unit volume which are conducive to higher discharge capacities.
As the hydrogen absorbing alloy for alkaline storage batteries, usually a hydrogen absorbing alloy powder prepared by solidifying a molten alloy in a mold with cooling water and grinding the solidified molding into a powder (this hydrogen absorbing alloy powder will hereinafter be referred to as the conventional solidification powder) was employed.
However, because the conventional solidification powder is rich in segregation (a variation in concentration of any alloying element), the alloy particles tend to crack as they absorb and release hydrogen in charging and discharging to undergo an increase in specific surface area. Therefore, alkaline storage batteries fabricated using the conventional solidification powder show satisfactory high-rate discharge characteristics in an early phase of cycling but because the loci of segregation tend to function as the origins of oxidative degradation (corrosion). Thus, they generally have the disadvantage of a short cycle life.
To improve the cycle life, it has been proposed to subject the conventional solidification powder to annealing (heat treatment) (Japanese Kokai Tokkyo Koho S60-89066). However, although the annealing of the conventional solidification powder prolongs the cycle life because of decreased segregation, the crystal size (the sum of the thicknesses of two adjacent layers in the lamellar type structure in which a layer rich in a specific element such as a rare earth element and a layer lean in the element alternate) is increased with the result that the particles become resistant to cleavage so that the high-rate discharge characteristic in an early phase of cycling, particularly the high-rate discharge characteristic at low temperature, deteriorates considerably as compared with the untreated powder.
In view of the above conflicting properties of the conventional solidification powder, a hydrogen absorbing alloy powder obtained by grinding a strip of hydrogen absorbing alloy prepared by the roll method (i.e. the method which comprises casting a molten alloy on the peripheral surface of a roll revolving at high speed and allowing it to be quenched in situ) has been proposed as a new hydrogen absorbing material (Japanese Kokai Tokkyo Koho S63-291367).
Since the hydrogen absorbing alloy produced by the roll method is obtained upon quenching of a molten alloy on a roll, the molten metal is least influenced by the gravitational field during solidification so that it features a reduced degree of segregation compared with the conventional solidification powder.
However, the roll side (the side held in contact with the peripheral surface of the roll) of the strip of hydrogen absorbing alloy produced by the roll method as well as the free side (the side opposite to said roll side of the strip of hydrogen absorbing alloy produced by the single-roll method) is not sufficiently microporous and has been more or less oxidized so that when the hydrogen absorbing alloy powder available upon grinding the strip is used directly as the hydrogen absorbing material, the high-rate discharge characteristic, particularly at low temperature, is poor owing to the low activity of the hydrogen absorbing alloy.